1. Field of the Invention
This invention relates to a method for production of an InGaAsP/InP type semiconductor laser having high output power and high operational reliability.
2. Discussion of Background
There has so far been known a high output power laser of an InGaAsP/InP type having a structure as shown in FIG. 5 of the accompanying drawing. In the drawing, a reference numeral 1 designates a p-type InP substrate, a numeral 2 refers to an n-type InP current blocking layer, a numeral 3 refers to a p-type InP current blocking layer, 4 denotes a p-type InP clad layer, 5 designates an InGaAsP active layer, 6 refers to an n-type InP clad layer, and 7 represents an n-type InGaAsP contact layer.
This laser is fabricated by the crystal growth for two times on the p-type InP substrate 1 having the crystallographic plane of (100). In the first crystal growth, the n-type InP current blocking layer 2, the p-type InP current blocking layer 3, and a thin layer of InGaAsP (not shown in the drawing) having a thickness of 0.2 .mu.m or so and to be used as an etching mask are grown on the p-type InP substrate in the order as mentioned. By the way, a p-type InP buffer layer (not shown in the drawing) is usually grown between the p-type InP substrate 1 and the n-type InP current blocking layer 2, this buffer layer being for reduction in influence of the strain onto the epitaxial layer of the p-type InP substrate 1.
In the next place, a stripe pattern having a width of about 1 to 2 .mu.m is formed along the &lt;011&gt; direction of the crystallographic plane (100) of the crystal which has completed its initial growth, and then a groove having a shape of arrowhead in its cross-section is formed with use of hydrochloric acid type etching solution and the InGaAsP layer as the mask, the tip end of the groove piercing through the n-type InP current blocking layer 2. Subsequently, as the second crystal growth, there are sequentially formed the p-type InP clad layer 4, the InGaAsP active layer 5, the n-type InP clad layer 6, and the n-type InGaAsP contact layer 7. The InGaAsP active layer 5 is grown in the arrowhead-shaped groove along the &lt;011&gt; direction of its crystallographic plane (100) in the cross-sectional shape of a crescent.
In this type of laser, the InGaAsP active layer 5 in the crescent shape is formed in the groove of a narrow width, and is enclosed by an InP crystal having its refractive index lower than that of the InGaAsP active layer 5, on account of which it constitutes a refractive index wave-guide type laser capable of performing stable fundamental transverse mode oscillation. Further, a reverse bias junction between the n-type InP current blocking layer 2 and the p-type InP current blocking layer 3, both being formed on the p-type InP substrate 1, has high breakdown voltage. Therefore, when this laser is biased in the forward direction to cause electric current to flow, it functions to suppress with extremely high effectiveness the leakage current flowing outside the InGaAsP active layer 5. As the result of this, the laser is capable of performing its oscillation at a low threshold value of from 10 to 20 mA or so, and also of carrying out a high output operation of 50 mW or more in its continuous operation. It is further excellent in its high temperature operation, i.e., it is capable of operating even at such a temperature as high as 100.degree. C. or above.
While the conventional high output power laser of the InGaAsP/InP type as described above is capable of performing the high output power operations, it has a point of problem such that, since the InGaAsP active layer 5 is exposed to the end face of the laser resonator, the InGaAsP active layer 5 tends to readily bring about oxidation on its surface with the consequence that its operational reliability at the high output power operation is not satisfactory.